Method for preparing asymmetric radial copolymers having two first arms and two second arms

ABSTRACT

A method for preparing asymmetric radial polymers wherein the different polymeric arms are contacted sequentially with a coupling agent. The method narrows the relative arm distribution of the several asymmetric radial polymers produced and significantly increases the amount of total product having the desired ratio of polymeric arms. Any coupling agent known in the prior art to be useful in the production of asymmetric radial polymers may be used in the method of this invention but coupling agents having from three to about twelve functional groups are most effective.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for preparing polymers. Moreparticularly, this invention relates to a method for preparingasymmetric radial polymers.

2. Prior Art

Heretofore, several methods have been proposed for preparing asymmetricradial polymers. As is well known in the prior art, radial polymerscomprise three or more arms extending outwardly from a nucleus. Theasymmetric radial polymers, generally, contain arms of at least twodifferent polymers, which polymers may vary as to chemical composition,structure and/or molecular weights. Asymmetric radial polymers havingarms of different molecular weights are sometimes referred to aspolymodal polymers. A principal difference in the methods frequentlyused to prepare both asymmetric and polymodal radial polymers resides inthe selection of a coupling agent which forms the nucleus of the radialpolymer. The coupling agent may contain a fixed, though sometimesvariable, number of functional sites such as the coupling agents taughtin U.S. Pat. Nos. 3,281,383; 3,598,884; 3,639,517; 3,646,161; 3,993,613and 4,086,298 or the coupling agent may itself be a monomer whichpolymerizes during the coupling reaction such as taught in U.S. Pat. No.3,985,830.

In general, and when an asymmetric polymer is prepared using one of themethods heretofore proposed, a blend of polymeric arms is first preparedcontaining the various polymeric arms in the desired ratio and the blendof polymeric arms is then added to the coupling agent or the couplingagent is added to the blend of polymeric arms. These methods do, then,result in the production of a product having, on average, the desirednumber of each kind of arm in the asymmetric polymer. The real problemassociated with producing asymmetric polymers in this fashion, however,is that the product obtained is in actuality a statistical distributionof all possible products which is represented by the equation: ##EQU1##for a polymer having the average composition (SI)_(x) (I)_(y) where eachpolymer component is designated as (SI₁)_(xi) (I₂)_(yi) wherein SIrepresents polystyrene-polyisoprene copolymer arms and I representspolyisoprene homopolymer arms on the radial polymer and quantitiesenclosed in brackets refer to molar concentrations.

For example, if one sought to produce an asymmetric radial polymerhaving three homopolymer arms and one copolymer arm using silicontetrachloride as the coupling agent by the methods heretofore proposed,a blend of polymeric arms comprising both living homopolymers and livingcopolymers in a ratio of three to one would be combined with the silicontetrachloride and the coupling reaction allowed to proceed tocompletion. The resulting asymmetric polymer would, of course, onaverage contain three homopolymer arms per copolymer arm. The actualproduct obtained would, however, be a blend of radial polymers, some ofwhich contain four homopolymer arms and no copolymer arms, some of whichcontain three homopolymer arms and one copolymer arm (the desiredproduct), some of which contain two homopolymer arms and two copolymerarms, some of which contain one homopolymer and three copolymer arms andsome of which contain no homopolymer arms and four copolymer arms. Theexpected statistical distribution for an asymmetric radial copolymerhaving the average composition (SI)-X-I₃ made in this manner, wherein Xis silicon, is given in Table 1. To the extent that an asymmetric radialcopolymer containing three homopolymer arms and one copolymer arm wasparticularly well suited for a particular end use application whileradial polymers containing less than three homopolymer arms were notparticularly well suited, the blend actually obtained would not, thenperform as well as desired in this particular end use application.

                  TABLE 1                                                         ______________________________________                                        Calculated Statistical Distribution of Polymer                                Components for the Asymmetric Radial Polymer Having a                         3:1 Arm Ratio                                                                 Polymer Component % Mole                                                      ______________________________________                                        (SI).sub.4 -X      0.4                                                        (SI).sub.3 -X-I    4.7                                                        (SI).sub.2 -X-I.sub.2                                                                           21.1                                                        (SI)-X-I.sub.3    42.2                                                        I.sub.4 -X        31.6                                                        ______________________________________                                    

Similarly, if one sought to produce an asymmetric radial polymer havingtwo homopolymer arms and two copolymer arms using silicon tetrachlorideas the coupling agent by the methods heretofore proposed, a blend ofpolymeric arms comprising both living homopolymers and living copolymersin a ratio of one to one would be combined with the silicontetrachloride and the coupling reaction allowed to proceed tocompletion. The resulting asymmetric polymer would, of course, onaverage contain two homopolymer arms and two copolymer arms. The actualproduct obtained would, however, be a blend of radial polymers, some ofwhich contain four homopolymer arms and no copolymer arms, some of whichcontain three homopolymer arms and one copolymer arm, some of whichcontain two homopolymer arms and two copolymer arms (the desiredproduct), some of which contain one homopolymer and three copolymerarms, and some of which contain no homopolymer arms and four copolymerarms. The expected statistical distribution for an asymmetric radialcopolymer having the average composition (SI)₂ I₂ made in this manner isgiven in Table 2. To the extent that an asymmetric radial copolymercontaining two homopolymer arms and two copolymer arm was particularlywell suited for a particular end use application while radial polymerscontaining less than two copolymer arms were not particularly wellsuited (since this type of radial polymer would not form a mechanicalnetwork and would thus have very low strength), the blend actuallyobtained would not, then perform as well as desired in this particularend use application.

                  TABLE 2                                                         ______________________________________                                        Calculated Statistical Distribution of Polymer                                Components for the Asymmetric Radial Polymer Having the                       Average Composition (SI).sub.2 I.sub.2                                        Polymer Component % Mole                                                      ______________________________________                                        (SI).sub.4        6.25                                                        (SI).sub.3 I      25                                                          (SI).sub.2 I.sub.2                                                                              37.5                                                        (SI)I.sub.3       25                                                          I.sub.4           6.25                                                        ______________________________________                                    

Recently, it has been discovered that narrower distributions of relativearm content in any given asymmetric radial polymer frequently does,indeed, lead to better performance in many end use applications. This isparticularly important when an asymmetric radial polymer containing acertain arm ratio may give rise to deleterious properties in anapplication. The need, then, for an improved process for preparingasymmetric radial polymers offering better control of the relativedistribution of polymer arms in the product is believed to be readilyapparent.

SUMMARY OF THE INVENTION

It has now been discovered that the foregoing and other disadvantages ofthe prior art processes for preparing asymmetric polymers can be avoidedor at least significantly reduced with the method for preparingasymmetric radial polymers of this invention and an improved process forpreparing asymmetric radial polymers provided thereby. It is therefore,an object of this invention to provide an improved process for preparingasymmetric radial polymers. It is another object of this invention toprovide such an improved process wherein the relative distribution ofarms within the polymer is controlled within a narrower range. It is yetanother object of this invention to provide such an improved processwherein the production of asymmetric radial polymers having relative armdistributions which are not beneficial in any given end use applicationare either eliminated or at least significantly reduced. The foregoingand other objects and advantages will become apparent from thedescription of the invention set forth hereinafter and from the examplesincluded therein.

In accordance with the present invention, the foregoing and otherobjects and advantages are accomplished with a process wherein thedifferent arms to be contained in the asymmetric radial polymer arecontacted sequentially with the coupling agent. Generally, the couplingsequence will be controlled by the relative number of each arm sought inthe final product with that polymer intended to provide the greaternumber of arms contacted with the coupling agent first and that polymerintended to provide the next greatest number of arms contacted with thecoupling agent second. To the extent that all of the arms are intendedto be present in the asymmetric radial polymer product in equal numbersthe order of addition is immaterial.

DETAILED DESCRIPTION OF THE INVENTION

As indicated supra, the present invention is drawn to an improvedprocess for making asymmetric radial polymers. As also indicated supra,asymmetric radial polymers contain a plurality (three or more) of armsof at least two different polymers. The polymeric arms may differ as tochemical composition, structure and/or molecular weight. In the processof the present invention, the different arms are contacted sequentiallywith the coupling agent. When the number of arms of one polymer isintended to be present in the asymmetric radial polymer product in agreater number than one or more other polymers, the polymer intended tobe present in the greater number will be contacted with the couplingagent first. It will of course be appreciated that the polymer intendedto provide the greater number of arms in the asymmetric polymer productcould be a mixture of different polymers. After reaction of the armsintended to be present in the greater number with the coupling agent iscomplete or at least substantially complete, the product therefrom willbe contacted with the arm intended to be present in the next greatestnumber and this reaction allowed to proceed until completed or at leastsubstantially completed. When two or more arms are intended to bepresent in equal number, the order of contacting with the coupling agentis not critical and each of the arms may be added in any order(sequence).

In general, the method of this invention may be used to prepareasymmetric radial polymers with any polymer containing a reactive endgroup which will react with one or more functional groups contained in aselected coupling agent. The method is particularly suitable for thepreparation of asymmetric radial polymers from so-called "living"polymers containing a single terminal metal ion. The coupling agent usedin the preparation must, then, contain at least three functional groupswhich will react with the polymer at the site of the metal ion. As iswell known in the prior art, "living" polymers are polymers containingat least one active group such as a metal atom bonded directly to acarbon atom. "Living" polymers are readily prepared via anionicpolymerization. Since the present invention is particularly well suitedto the preparation of asymmetric radial polymers using "living" polymersto form the arms thereof, the invention will be described by referenceto such polymers. It will, however, be appreciated that the inventionwould be equally useful with polymers having different reactive groupsso long as the selected coupling agent contains functional groups whichare reactive with the reactive site contained in the polymer.

Living polymers containing a single terminal group are, of course, wellknown in the prior art. Methods for preparing such polymers are taught,for example, in U.S. Pat. Nos. 3,150,209; 3,496,154; 3,498,960;4,145,298 and 4,238,202, the disclosure of which patents are hereinincorporated by reference. In general, the polymers produced with theprocesses taught in the foregoing patents may be polymers of one or moreconjugated dienes containing from 4 to about 12 carbon atoms such as1,3-butadiene, isoprene, piperylene, methylpentadiene, phenylbutadiene,3,4-dimethyl-1,3-hexadiene, 4,5-diethyl-1,3-octadiene and the like,preferably those conjugated diolefins containing 4 to 8 carbon atoms. Inaccordance with the disclosure of at least certain of these patents, oneor more of the hydrogen atoms in the conjugated diolefins may besubstituted with halogen. The polymers produced by these processes mayalso be copolymers of one or more of the aforementioned conjugateddiolefins and one or more other monomers particularly monoalkenylaromatic hydrocarbon monomers such as styrene, various alkyl-substitutedstyrenes, alkoxy-substituted styrenes, vinyl naphthalene, vinyl tolueneand the like. Homopolymers and copolymers of monoalkenyl aromatichydrocarbons can also be prepared by the methods taught in theaforementioned patents, particularly the methods taught in U.S. Pat.Nos. 3,150,209; 3,496,154; 3,498,960; 4,145,298 and 4,238,202. When thepolymer product is a random or tapered copolymer, the monomers are,generally, added at the same time, although the faster reacting monomermay be added slowly in some cases, while, when the product is a blockcopolymer, the monomer used to form the separate blocks are addedsequentially.

In general, the polymers useful as arms in both the method of thisinvention and the asymmetric radial polymer of this invention may beprepared by contacting the monomer or monomers with an organoalkalimetal compound in a suitable solvent at a temperature within the rangefrom about -150° C. to about 300° C., preferably at a temperature withinthe range from about 0° C. to about 100° C. Particularly effectivepolymerization initiators are organolithium compounds having the generalformula:

    RLi

Wherein:

R is an aliphatic, cycloaliphatic, alkyl-substituted cycloaliphatic,aromatic or alkyl-substituted aromatic hydrocarbon radial having from 1to about 20 carbon atoms. In general, the polymers useful as arms inboth the method of this invention and the asymmetric radial polymer ofthis invention will have a weight-average molecular weight within therange from about 1000 to about 500,000 and when the polymer is acopolymer of one or more conjugated diolefins and one or more othermonomers, the copolymer will comprise from about 1 wt % to about 99 wt %monomeric diolefin units and from about 99 wt % to about 1 wt %monoalkenyl aromatic hydrocarbon monomer units. In general, thedifferent polymer arms will be prepared separately and maintainedseparately until each is sequentially contacted with the coupling agent.

In general, any of the coupling agents known in the prior art to beuseful in forming a radial polymer by contacting the same with a livingpolymer may be used in both the method of this invention and theasymmetric radial polymers of this invention. In general, suitablecoupling agents will contain three or more functional groups which willreact with the living polymer at the metal-carbon bond. While the methodof the present invention will, at least, improve the relativedistribution of different arms in an asymmetric radial polymer havingany number of arms, the method offers significant improvement when thecoupling agent contains from three to about twelve functional groupsreactive with the metal-carbon bond of the "living" polymer. Suitablecoupling agents, then, include SiX₄, RSiX₃, HSiX₃, RX₂ Si--(CH₂)_(x)--SiX₂ R, X₃ Si--SiX₃, X₃ Si--O--SiX₃, X₃ Si--(CH₂)_(x) --SiX₃, RX₂Si--(CH₂)_(x) --SiX₂ --(CH₂)_(x) --SiX₂ R, R--C(SiX₃)₃, R--C(CH₂ SiX₃)₃,C(CH₂ SiX₃)₄ and the like, particularly those containing from three toabout six functional groups. In the foregoing formulae: each X may,independently, be fluorine, chlorine, bromine, iodine, alkoxideradicals, carboxylate radicals, hydride and the like; R is a hydrocarbylradical having from 1 to about 10 carbon atoms, preferably from 1 toabout 6 carbon atoms; and x is a whole number from 1 to about 6.Particularly useful coupling agents include the silicon tetrahalidessuch as silicon tetrafluoride, silicon tetrachloride, silicontetrabromide and the like.

In general, the living polymers used as arms in the asymmetric radialpolymer will be contacted with the coupling agent at a temperaturewithin the range from about 0° C. to about 100° C. at a pressure withinthe range from about 0 psig to about 100 psig and the contacting in eachstep will be maintained until reaction between the arms and the couplingagent is complete, generally for a period of time within the range fromabout 1 to about 180 minutes. While the inventor does not wish to bebound by any particular theory, it is believed that the method of thisinvention results in an improved relative arm distribution because thereactivity of the functional groups contained in the coupling agentbecome progressively less active as the number of functional groupscontained in the coupling agent is reduced as the result of reactionwith the metal-carbon bond contained in the living polymer. Further, itis believed that this reduction in activity is caused primarily bysteric hinderance resulting from the progressive incorporation ofpolymer segments onto the coupling agent. The last functional siteremaining on the coupling agent would, then, be the least reactive nomatter how many functional groups the coupling agent initiallycontained.

When the stoichiometry of the first polymeric arms to be added relativeto the coupling agent is such that there is enough polymer arms to reactwith all but one functional group on each silicon atom, then a polymericintermediate with a precise structure is formed. When the second portionof arms is contacted with the partially reacted coupling agent, thesecond arms will react with the remaining functional groups (one on eachsilicon atom) giving a precise asymmetric radial polymer. Thisdifference in reactivity, then, when coupled with sequential addition ofthe different arms in controlled stoichiometric amounts ensures thateach asymmetric radial polymer formed will have the desired number ofeach arm.

When the stoichiometry of the first polymer arms to be added relative tothe coupling agent is such that there are less than enough polymer armsthan that required to react with all but one functional group on eachSilicon atom of the coupling agent (but more than one polymer arm perSilicon), then a distribution of partially coupled polymeric productswill be obtained. However, since the last functional group is slower toreact, as discussed above, there will not be any coupling agentmolecules that have all of their sites reacted with polymer forming asymmetric radial polymer containing only arms of the first polymer type.

In addition, since polymer chain ends are very reactive towards thefunctional groups on the Silicon atoms of the coupling agent, there willnot be any Silicon that do not have at least one polymeric arm. When thenext portion of polymeric arms is contacted with the partially reactedcoupling agent, they will react with the remaining functional groupsgiving an asymmetric radial polymer distribution which is significantlynarrower than that obtained by the prior art methods and which will notcontain any non-asymmetric radial polymer composed only of one polymericarm type. This would be the synthetic method used when an asymmetricradial polymer is desired such that a precisely controlled ratio ofdiffering polymer arms is not critical to the performance of thepolymer, but a narrower distribution (containing significantly more ofthe desired asymmetric radial polymer without the presence ofnon-asymmetric radial polymers) than is available using the prior artmethods is desired for improved performance in the application isneeded.

In addition, the method of this invention allows for the use of moreeconomical, commercially available coupling agents. For example, aprecise asymmetric radial polymer containing two copolymer arms and twohomopolymer arms may be synthesized using the first described method ifthe coupling agent RCl₂ Si(CH₂)₂ SiCl₂ R is used. However, SiCl₄ may beused in the second described method to produce an asymmetric radialpolymer having on average two copolymer arms and two homopolymer arms.In this case, the product will, in actuality, be a mixture of asymmetricradial polymers, some having three copolymer arms and one homopolymerarm, the majority having two copolymer arms and two homopolymer arms,and some having one copolymer arm and three homopolymer arms. Unlike theproduct obtained using the prior art methods, the product will notcontain symmetric radial polymers containing four copolymer arms and nohomopolymer arms, or four homopolymer arms and no copolymer arms.

For applications that can tolerate a distribution of products but cannottolerate the presence of symmetric radial polymers containing allcopolymer arms and no homopolymer arms and symmetric radial polymerscontaining all homopolymer arms and no copolymer arms the method of thisinvention provides a simple, economical method to obtain significantlyhigher amounts of the desired product while avoiding the presence ofundesired radial polymers. The method of the present invention isparticularly useful for the production of an asymmetric polymer whereinequal numbers of each different polymer arm type are desired. When allof the polymer arms are combined and contacted simultaneously with thecoupling agent, on the other hand, the distribution of the differentarms on each of the asymmetric radial polymers formed will bestatistical and can range from polymers having all arms of one type ofpolymer to all arms of another type of polymer as shown in Tables 1 and2.

In general, the polymers useful as arms in the asymmetric radialpolymers of this invention will be in solution when contacted with thecoupling agent. Suitable solvents include those useful in the solutionpolymerization of the polymer and include aliphatic, cycloaliphatic,alkyl-substituted cycloaliphatic, aromatic and alkyl-substitutedaromatic hydrocarbons, ethers and mixtures thereof. Suitable solvents,then, include aliphatic hydrocarbons such as butane, pentane, hexane,heptane and the like, cycloaliphatic hydrocarbons such as cylohexane,cycloheptane and the like, alkyl-substituted cycloaliphatic hydrocarbonssuch as methylcyclohexane, methylcycloheptane and the like, aromatichydrocarbons such as benzene and alkyl-substituted aromatic hydrocarbonssuch as toluene, xylene and the like and ethers such as tetrahydrofuran,diethylether, di-n-butyl ether and the like. Since the polymers usefulin making the asymmetric radial polymers of this invention will containa single terminal reactive group, the polymers used in preparation ofthe asymmetric radial polymers will be retained in solution afterpreparation without deactivating the reactive (living) site. In general,the coupling agent may be added to a solution of the polymer or asolution of the polymer may be added to the coupling agent.

The method of preparing the asymmetric radial polymers of this inventionwill comprise a plurality of steps. In the first step, a polymercontaining a single terminal reactive group will be contacted with acoupling agent containing a plurality of functional groups which arereactive with the terminal group of the polymer. In a second step, thereaction product from the first step will be combined with a solution ofa second polymer which is different from the polymer used in the firststep. The difference may be in chemical composition, relative chemicalcomposition, structure, molecular weight or the like. In the secondstep, contacting between the second polymer and the reaction productfrom the first step will be continued until reaction between the secondpolymer and the remaining functional groups of the coupling agent iscomplete. In all except the last step of the preparation, it will beimportant to control the amount of polymer contacted with the couplingagent such that, on average, the desired number of such arms areincorporated into the nucleus of each of the asymmetric radial polymersactually formed. In the final step, careful control of the amount ofpolymer used is not as important so long as a sufficient amount ofpolymer to react with all of the remaining functional groups in thecoupling agent is used. To the extent that the polymer used is notreadily separable from the asymmetric radial polymer, however, and tothe extent that the presence of such a polymer in the final product isundesirable, care should be exercised to ensure that a stoichiometricamount of the last polymer, relative to the remaining functional groups,is used.

The asymmetric radial polymers of this invention may be used in any ofthe applications for which asymmetric radial polymers having the sameaverage relative arm structure can be used. Suitable end useapplications, then, include impact modification of engineeringthermoplastics, impact modification of unsaturated thermosettingpolyesters, asphalt modification, viscosity index improvers, adhesivesand the like.

PREFERRED EMBODIMENTS

In a first preferred embodiment of the present invention, the process ofthis invention will be used to prepare an asymmetric radial polymerhaving four arms. The arms will be, partly, polymers containing onlyconjugated diolefins, most preferably conjugated diolefin homopolymersand, partly, block copolymers containing at least one monoalkenylaromatic hydrocarbon polymer block and at least one conjugated diolefinpolymer block. In a most preferred embodiment, the block copolymer willcomprise a single polystyrene block and a single polybutadiene orpolyisoprene block. The weight-average molecular weight of thosepolymeric arms containing only polymerized conjugated diolefins will bewithin the range from 1,000 to about 150,000, preferably from about15,000 to about 150,000. The weight-average molecular weight of themonoalkenyl aromatic hydrocarbon polymer blocks will be within the rangefrom about 5,000 to about 100,000 and the weight-average molecularweight of the conjugated diolefin polymer blocks will be within therange from about 15,000 to about 150,000. Both the conjugated diolefinpolymer arms and the block copolymer arms will be living polymerscontaining a single lithium atom bonded to a terminal carbon atom. Inthe first preferred embodiment, any of the known coupling agentscontaining four functional groups which are reactive with thelithium-carbon bond may be used. In a most preferred embodiment, thecoupling agent will be silicon tetrachloride. In a most preferredembodiment of the present invention, the ratio of conjugated diolefinhomopolymer arms to the styrene-butadiene or styrene-isoprene blockcopolymer arms will be 3:1.

In the first preferred embodiment, and when the asymmetric radialpolymer is intended to contain an average of three of one type ofpolymer arms to one of the other, the polymer intended to constitute thethree arms will be contacted with the coupling agent first and thereaction between the lithium-carbon bond and the functional groupsallowed to proceed to completion. When the polymer is intended tocontain two of both kinds of arms, either polymer may be first contactedwith the coupling agent. In the first preferred embodiment, thesequential coupling reactions will be completed at a temperature withinthe range from 20° C. to about 80° C., preferably from about 50° C. toabout 80° C., at a pressure within the range from about 0 psig to about30 psig with a nominal holding time within the range from 10 to about100 minutes, preferably from about 20 to about 100 minutes.Stoichiometric quantities of all reactants will be used in each step.

In a second preferred embodiment of the present invention, the processof this invention is used to prepare a four-arm radial polymer having aratio of first arms to second arms of 2:2. The arms are, partly,polymers containing only conjugated diolefins, most preferablyconjugated diolefin homopolymers and, partly, block copolymerscontaining at least one monoalkenyl aromatic hydrocarbon polymer blockand at least one conjugated diolefin polymer block. In a most preferredembodiment, the block copolymer will comprise a single polystyrene blockand a single polybutadiene or polyisoprene block. The weight-averagemolecular weight of those polymeric arms containing only polymerizedconjugated diolefins will be within the range from 1,000 to about150,000, preferably from about 15,000 to about 150,000. Theweight-average molecular weight of the monoalkenyl aromatic hydrocarbonpolymer blocks will be within the range from about 5,000 to about100,000 and the weight-average molecular weight of the conjugateddiolefin polymer blocks will be within the range from about 15,000 toabout 150,000. Both the conjugated diolefin polymer arms and the blockcopolymer arms will be living polymers containing a single lithium atombonded to a terminal carbon atom. In the second preferred embodiment,any of the known coupling agents containing four functional groups whichare reactive with the lithium-carbon bond may be used. In a mostpreferred embodiment, the coupling agent will be silicon tetrachloride.

In the second preferred embodiment, where the polymer is intended tocontain two of both kinds of arms, either polymer may be first contactedwith the coupling agent. In the second preferred embodiment, thesequential coupling reactions will be completed at a temperature withinthe range from 20° C. to about 80° C., preferably from about 50° C. toabout 80° C., at a pressure within the range from about 0 psig to about30 psig with a nominal holding time within the range from 10 minutes toabout 100 minutes, preferably from about 20 to about 100 minutes.Stoichiometric quantities of all reactants will be used in each step.Ethers such as glyme, diethylether, dimethoxybenzene, or tetramethyleneethylenediamine are added before the second coupling step to acceleratethe reaction between the last halogen on each silicon atom and theterminal functional group on the second polymeric arms. Stoichiometricquantities of all reactants are used in each step.

Having thus broadly described the present invention and a preferred andmost preferred embodiments thereof, it is believed that the inventionwill become even more apparent by reference to the following examples.It will be appreciated, however, that the examples are presented solelyfor the purposes of illustration and should not be construed as limitingthe invention.

EXAMPLES Example 1

In this example, an asymmetric radial polymer within the scope of thepresent invention containing, on average, three homopolymer arms and oneblock copolymer arm was prepared. The homopolymer arm was apolybutadiene having a weight-average molecular weight of 44,800. Theblock copolymer was a block copolymer comprising a single polystyreneblock having a weight-average molecular weight of 18,000 and a singlepolybutadiene block having a weight-average molecular weight of 41,600.In the first step of the preparation, a sufficient amount of a livingpolybutadiene polymer to provide three moles of living polymer per moleof silicon tetrachloride was contacted with silicon tetrachloride at atemperature of 60° C. and at ambient pressure. The living polymer wasdissolved in cyclohexane at a concentration of 12 wt % of polybutadieneand the contacting was accomplished by adding the silicon tetrachlorideto the polymer solution. The contacting was maintained for 30 minuteswith mild agitation. After reaction of the living polybutadiene polymerand the silicon tetrachloride was complete, a sufficient amount of aliving styrene-butadiene block copolymer was added to the solution toprovide one mole of block copolymer per mole of silicon tetrachlorideinitially in solution. Contacting between the block copolymer and thereaction product from the first step was continued for 30 minutes at thesame conditions used during the first contacting step. After reaction ofthe block copolymer and the coupling agent was completed, the asymmetricradial polymer was recovered and analyzed to determine the relativeamount of radial polymer containing no homopolymer arms, one homopolymerarm, two homopolymer arms, three homopolymer arms and four homopolymerarms. The results obtained are summarized in the Table 3 which comparesthe results to the calculated distribution from Table 1.

                  TABLE 3                                                         ______________________________________                                                       % of Radial Polymer With #                                                    of Homopolymer Arms                                                           Indicated                                                      Number of        Method of  Prior Art                                         Homopolymer Arms this Invention                                                                           Method                                            ______________________________________                                        0                0          <1                                                1                0           5                                                2                5          21                                                3                86         42                                                4                9          32                                                ______________________________________                                    

As will be apparent from the data summarized in Table 3, the method ofthis invention more than doubled the amount of polymer produced havingthe desired ratio of arms; viz., three homopolymer arms and onecopolymer arm (86% vs. 42%). As will also be apparent from the datasummarized in the preceding Table, the method of this invention resultedin a product containing only 5% of polymer molecules having two or morecopolymer arms while the prior art method results in a productcontaining greater than 26% of polymer molecules having two or morecopolymer arms. It is important, particularly in some impactmodification end uses, that polymers containing two or more copolymerarms be minimized. The method of the present invention is, then, quiteeffective in narrowing the relative distribution of the arms in theasymmetric radial polymer produced.

Example 2

In this example, an asymmetric radial polymer within the scope of thepresent invention containing two isoprene homopolymer arms and twostyrene-butadiene block copolymer arms was prepared. The polyisoprenearms had a weight-average molecular weight of 18,000. The blockcopolymer arms had a polystyrene block weight-average molecular weightof 10,400 and a polybutadiene block weight-average molecular weight of24000. In the first step of the preparation, a sufficient amount of aliving isoprene homopolymer to provide two moles of living polymer permole of silicon tetrachloride was contacted with silicon tetrachlorideat a temperature of 60° C. and at ambient pressure. The living polymerwas dissolved in cyclohexane at a concentration of 20 wt % ofpolyisoprene and the contacting was accomplished by adding the silicontetrachloride to the polymer solution. The contacting was maintained for60 minutes with mild agitation. Separately, a styrene-butadiene blockcopolymer was synthesized in the presence of 6% diethylether on a totalsolution basis which resulted in a 41% 1,2-butadiene content asdetermined by 1H NMR analysis. After reaction of the living isoprenehomopolymer and the silicon tetrachloride was complete, a sufficientamount of the living styrene-butadiene block copolymer and about 300 ppmglyme on a total solution basis was added to the solution to provide twomoles of block copolymer per mole of silicon tetrachloride initially insolution. Contacting between the block copolymer and the reactionproduct from the first step was continued for 60 minutes at the sameconditions used during the first contacting step. After reaction of theblock copolymer and the coupling agent was completed, the asymmetricradial polymer was recovered.

The asymmetric radial polymer was then partially hydrogenated using anickel-aluminum catalyst under conditions that do not hydrogenatearomatic double bonds and will preferentially hydrogenate polybutadienedouble bonds rather than polyisoprene double bonds.

Example 3

In this example, an asymmetric radial polymer within the scope of thepresent invention containing two isoprene homopolymer arms and twostyrene-isoprene block copolymer arms was prepared according to theprocedure of Example 2 with the following changes. The isoprenehomopolymer arms had a weight-average molecular weight of 9,200. Thestyrene-isoprene block copolymer arms had a polystyrene blockweight-average molecular weight of 6,300 and a polyisoprene blockweight-average molecular weight of 22,000. In the first coupling step,the living polyisoprene was contacted with the silicon tetrachloride at25° C. for 60 minutes. In the second coupling step, a sufficient amountof living styrene-isoprene block polymer, was added to the solution toprovide two moles of block polymer per mole of silicon tetrachloride.After reaction of the block polymer and the coupling agent was complete,the asymmetric radial polymer was recovered.

The asymmetric radial polymer was fully hydrogenated using anickel-aluminum catalyst under conditions that do not hydrogenatearomatic double bonds and will hydrogenate the polyisoprene doublebonds.

Example 4

In this example, an asymmetric radial polymer within the scope of thepresent invention containing two isoprene homopolymer arms and twostyrene-butadiene block copolymer arms was prepared according to theprocedure of Example 2 with the following changes. The polyisoprene armshad a weight-average molecular weight of 18,400. The styrene-butadieneblock copolymer arms had a polystyrene block weight-average molecularweight of 5,500 and a polybutadiene block weight-average molecularweight of 19,400. In the first coupling step, the living polyisoprenewas contacted with the silicon tetrachloride at 25° C. for 60 minutes.The second coupling step was conducted at 70° C. for 60 minutes in thepresence of 300 ppm glyme on a total solution basis. After reaction ofthe block polymer and the coupling agent was complete, the asymmetricradial polymer was recovered. The 1,2-butadiene content of the resultingpolymer was 39.6% as determined by ¹ H NMR analysis.

A portion of this asymmetric radial polymer was partially hydrogenatedas indicated in Example 2 and another portion was fully hydrogenated asindicated in Example 3.

Example 5

In this example, an asymmetric radial polymer within the scope of thepresent invention containing two isoprene homopolymer arms and twostyrene-butadiene block copolymer arms was prepared according to theprocedure of Example 2 with the following changes. The polyisoprene armshad a weight-average molecular weight of 19,600. The styrene-butadieneblock copolymer arms had a polystyrene block weight-average molecularweight of 5,800 and a polybutadiene block weight-average molecularweight of 21,500. The second coupling step was conducted at 70° C. for60 minutes. After reaction of the block polymer and the coupling agentwas complete, the asymmetric radial polymer was recovered. The1,2-butadiene content of the resulting polymer was 40% as determined by¹ H NMR analysis.

A portion of this asymmetric radial polymer was partially hydrogenatedas indicated in Example 2 and another portion was fully hydrogenated asindicated in Example 3.

For examples 2, 3, 4, and 5, above, after reaction of the livingisoprene homopolymer and the silicon tetrachloride was complete, analiquot of the solution was removed and analyzed by Gel PermeationChromatography to determine the relative amounts of coupling agentcontaining one, two and three polymer arms. This analysis allowsprediction of the final polymer composition after the second couplingstep is completed and in some cases is the only available method ofdetermination since, depending upon the relative polymer arm weightaverage molecular weights, Gel Permeation Chromatography does not allowresolution of the various polymer components of the final product. Theresults of this analysis establish that the method of this invention issuccessful in producing a significantly narrower distribution of polymerproducts by complete elimination of symmetric radial polymers (thosecontaining four homopolymer arms and no block copolymer arms or fourblock copolymer arms and no homopolymer arms). The prior art methodswould result in a product containing 40%wt of these non-asymmetricradial polymers, and only 20%wt of the desired asymmetric radial polymerhaving two homopolymer arms and two block copolymer arms. The method ofthis invention is, then, very effective in narrowing the distribution ofarms in the asymmetric radial polymer produced.

While the present invention has been described and illustrated byreference to particular embodiments thereof, it will be appreciated bythose of ordinary skill in the art that the same lends itself tovariations not necessarily described or illustrated herein. For thisreason, then, reference should be made solely to the appended claims forpurposes of determining the true scope of the present invention.

Having thus described and illustrated the invention; what is claimedis:
 1. A method for preparing an asymmetric radial polymer comprisingthe steps of: p1 (a) contacting a first conjugated diene polymer havinga single reactive end group per molecule with a coupling agentcontaining four functional groups which will react with the reactive endgroup such that said first conjugated diene polymer reacts with two ofthe functional groups on each molecule of the coupling agent;(b)contacting a second conjugated diene polymer having a single reactiveend group per molecule with the reaction product from step (a) underconditions such that reaction between said second polymer and thereaction product from step (a) proceeds substantially to completion; and(c) recovering an asymmetric radial polymer having four-arm radialpolymer molecules having a ratio of two polymeric arms from the firstconjugated diene polymer to two polymeric arms from the secondconjugated diene polymer.
 2. The method of claim 1 wherein the couplingagent comprises four halogen functional groups.
 3. The method of claim 2wherein the first conjugated diene polymer comprises living isoprenehomopolymer and the second conjugated diene polymer comprises livingpolystyrene-polyisoprene or living polystyrene-polybutadiene diblockcopolymers.
 4. The method of claim 3 wherein said coupling agentcontains four chlorine functional groups.
 5. The method of claim 4wherein said coupling agent is silicon tetrachloride.
 6. The method ofclaim 5 wherein the living isoprene homopolymers have weight averagemolecular weights from 1,000 to 150,000, the living block copolymershave weight average molecular weights from 1,000 to 150,000.
 7. Anasymmetric radial polymer prepared with a method comprising the stepsof:(a) contacting a first conjugated diene polymer having a singlereactive end group per molecule with a coupling agent containing fourfunctional groups which will react with the reactive end group such thatsaid first conjugated diene polymer reacts with two of the functionalgroups on each molecule of the coupling agent; (b) contacting a secondconjugated diene polymer having a single reactive end group per moleculewith the reaction product from step (a) under conditions such thatreaction between said second polymer and the reaction product from step(a) proceeds substantially to completion; and (c) recovering anasymmetric radial polymer having four-arm radial polymer moleculeshaving a ratio of two polymeric arms from the first conjugated dienepolymer to two polymeric arms from the second conjugated diene polymer.8. The asymmetric radial polymer of claim 7 wherein the coupling agentcomprises four halogen functional groups.
 9. The asymmetric radialpolymer of claim 8 wherein the first conjugated diene polymer comprisesliving isoprene homopolymer and the second conjugated diene polymercomprises living polystyrene-polyisoprene or livingpolystyrene-polybutadiene diblock copolymers.
 10. The asymmetric radialpolymer of claim 9 wherein said coupling agent contains four chlorinefunctional groups.
 11. The asymmetric radial polymer of claim 10 whereinsaid coupling agent is silicon tetrachloride.
 12. The asymmetric radialpolymer of claim 11 wherein the living isoprene homopolymers have weightaverage molecular weights from 1,000 to 150,000, the living blockcopolymers have weight average molecular weights from 1,000 to 150,000.